atomic_add_value()

Safely add to a variable, returning the previous value

Synopsis:

#include <atomic.h>

unsigned atomic_add_value( volatile unsigned * loc,
                           unsigned incr );

Arguments:

loc
A pointer to the value that you want to add to.
incr
The number that you want to add.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The atomic_add_value() function is a thread-safe way of doing an (*loc) += incr operation, even in a symmetric-multiprocessing system.

The atomic_*() functions are guaranteed to complete without being preempted by another thread.

When modifying a variable shared between a thread and an interrupt, you must either disable interrupts or use the atomic_*() functions.

The atomic_*() functions are also useful for modifying variables that are referenced by more than one thread (that aren't necessarily in the same process) without having to use a mutex.


Note: The atomic_add_value() function may be slower than atomic_add().

Returns:

The previous value of loc's contents.

Examples:

To safely increment a counter shared between multiple threads:

#include <atomic.h>
...

volatile unsigned count;
unsigned previous;
...

previous = atomic_add_value( &count, 1 );

Classification:

QNX Neutrino

Safety:
Cancellation point No
Interrupt handler Yes
Signal handler Yes
Thread Yes

See also:

atomic_add(), atomic_clr(), atomic_clr_value(), atomic_set(), atomic_set_value(), atomic_sub(), atomic_sub_value(), atomic_toggle(), atomic_toggle_value()